Floating roof for liquid storage tanks, particularly for the storage of liquid petroleum products

ABSTRACT

This disclosure relates to a large diameter floating roof for liquid storage tanks which comprises a covering sheetlike member and a peripheral hollow annular pontoon member fixed on the periphery of said sheetlike member and at least one intermediate annular concentric load member arranged to separate a central zone from a peripheral zone of said sheetlike member between said peripheral pontoon member and said intermediate load member.

iinited States Belleli [4 Mar.7,i917

[54] FLOATKNG ROOF FOR LIQUID STORAGE TANKS, PARTICULARLY FOR THE STORAGE O1" LKQUHD ?ETROLEUM PRODUCTS 211 Appl. No.: 854,540

[30] Foreign Application Priority Data Sept. 4, 1968 Italy ..20841 M68 [52] US. Cl. ..220/26 D [51] Int. Cl ..B65d 87/18 [58] Field of Search "220/26, 26 S, 26 S1, 26 D [56] References Cited UNITED STATES PATENTS 1,493,091 5/1924 Wiggins ..220/26 D 2,282,772 5/1942 Wiggins... ...220/26 R 2,282,773 5/1942 Wiggins ..220/26 S 2,287,213 6/1942 Wiggins ..220/26 R 2,359,416 10/ 1944 l-lammeren ....220/26 R 2,464,803 3/ 1949 Goldsby et al. ....220/26 R 2,614,717 10/1052 Wiggins ....220/26 D 2,635,782 4/1953 Orr ....220/26 R 2,750,067 6/1956 Wiggins... ....220/26 R 2,848,799 8/ 1958 Hanna ..220/26 R FOREIGN PATENTS OR APPLICATIONS 222,957 7/1959 Australia ..220/26 D Primary ExaminerJoseph R. Leclair Assistant ExaminerJames R. Garrett At!orneyGuido Modian'o and Albert Josif [57] ABSTRACT This disclosure relates to a large diameter floating roof for liquid storage tanks which comprises a covering sheetlike member and a peripheral hollow annular pontoon member fixed on the periphery of said sheetlike member and at least one intermediate annular concentric load member arranged to separate a central zone from a peripheral zone of said sheetlike member between said peripheral pontoon member and said intermediate load member.

1 Claims, 7 Drawing Figures PATENTEUMAR 7 I972 INVENTOR RODOLFO BELLELI Aqnnt PATENTEBHAR 7 1972 SHEET 2 0F 2 INVENTOR FLOATING ROOF FOR LIQUID STORAGE TANKS, PARTICULARLY FOR THE STORAGE OF LIQUID PETROLEUM PRODUCTS BACKGROUND OF THE INVENTION This invention relates to a floating roof for liquid storage tanks and more particularly relates to tanks of large dimensions (large diameter) intended for the storage of liquid petroleum products. It is well known that floating roofs of the above-mentioned type comprise a large-diameter circular roof plate of small thickness, joined at its periphery to an annular pontoon member and that such a roof plate is formed from a plurality of plates welded together. It is also known to the technicians of this field that such large diameter roof plates, irrespective of the thickness of the plates from which they are formed, behave substantially as relatively flexible membranes which deform under the action of external stresses, such as gusts of wind, weight of snow, rain and other accidental overloads. Such deformation may give rise to fatigue stress at the welds and cause rapid fracturing of some of the welds.

Another technical problem relative to the installation of floating roofs consists in the possibility of deformation of the tank shell (ovalization) due to the settling of foundations or other causes, which is a possibility which is especially serious in the case of large diameter tanks (60-90 meters and more). A corresponding elastic deformation of the floating roof at the peripheral pontoon member must accompany this ovalization in order to prevent possible jamming of the roof with consequent nullifying of its vertical movements on the liquid surface.

The peripheral deformations of floating roofs occur satisfactorily when the peripheral pontoon member has a small transverse section, but in such case the buoyancy is not sufficient for large diameter or simple pontoon roofs.

Moreover, a simple pontoon roof must have a peripheral pontoon member of large section which is particularly stiffened to counteract the radial forces which arise whenever the central roof is not sustained by the underneath liquid when it is flooded.

A further problem which has been unsatisfactorily solved is that of drainage. In fact the surface over which the water must run is too large to allow sufficient evacuation.

It is thus clear that the technical problem in question requires a solution which simultaneously takes account of peripheral deformation requirements, of floating, of resistance to radial stress and of good water drainage. The present tendency in the art towards the adoption of peripheral pontoon members of large transverse section,v thus resolving the requirements of floating and resistance, are not satisfactory from the point of view of peripheral deformation, without the expense of special arrangements, and do not resolve the problem of drainage.

The main object of this invention is that of providing a floating roof of large dimensions (large diameter) having a structure which simultaneously satisfies the technical problems expounded above, i.e., a floating roof of large dimensions which is designed in accordance with prescribed technical standards for this type of construction, which has the central roof plate stiffened against dynamic stresses produced by the wind, which is endowed overall with a satisfactory peripheral deformability" so as to avoid the danger of jamming in the case of ovalization of the tank for which the roof is intended.

In view of the zones of depression which form usually in roofs of great dimensions, due to the irregular stresses and also SUMMARY OF THE INVENTION According to the invention a large diameter floating roof for liquid storage tanks, particularly for the storage of liquid petroleum products, includes a sheetlike member extending substantially over the entire roof area and a peripheral hollow annular pontoon member fixed on the periphery of said sheet member and is characterized in that it comprises further at least one intermediate hollow annular member substantially concentric with the peripheral pontoon member, said intermediate hollow annular member separating the draining area of the floating roof into at least one zone arranged on one side of said annular member and at least another zone arranged on the other side of said annular member.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will appear more clearly from the following detailed description of a preferred embodiment of a floating roof according to the invention, illustrated by way of example in the accompanying drawings, in which:

FIG. I is a plan view of a floating roof according to the invention including a central floating caisson member,

FIG. 2 is a sectional view of the same floating roof through its diameter when attached to a tank for the storage of liquid products, but showing a modification without the central floating caisson member,

FIG. 3 is an enlarged partial view through the diameter of the peripheral pontoon member of the floating roof according to the invention,

FIG. 4 is an enlarged partial sectional view of the intermediate annular member of the floating roof according to the invention taken through its diameter;

FIG. 5 is a constructive detail of the said floating roof according to the invention,

FIG. 6 is a sectional view of a modification of the intermediate annular member and FIG. 7 shows a partial section view of the central zone of the roof according to a modified embodiment including a central floating caisson.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the above Figures, the large diameter floating roof according to the invention, which is fonned of a plurality of sheet metal panels or plates 1 welded together, comprises a peripheral pontoon member 2, divided internally into a plurality of scaled caisson chambers 3, and an intermediate load member 4 of a hollow annular fonn, fixed on the upper surface of the said roof (FIGS. 1 and 4) and positioned at a distance from the center of the said roof so that it subdivides the floating roof into a central zone 6 and a peripheral zone 7 which are separated from each other from a draining point of view and normally do not communicate.

According to one of the embodiments, the central floating caisson 5 may be omitted.

The behavior of the roof will now be considered under different working conditions as far as the embodiment without the central floating caisson is considered.

Under normal conditions, owing to the particular reinforced membrane structure, which will be more fully described later, and particularly as a consequence of the additional weight of the intermediate annular member, the zone nearest to this member assumes a level lower than the other zones of the roof, considering that the central zone 6 floats on the underlying liquid even without the central caisson 5, in the same manner as the entire roof floats. Consequently, the rain water or other liquids on the roof flow towards said lowermost zone from the central part of the roof and from the peripheral part thereof, so that the distance travelled by the water is reduced to a half, draining channels and pipes being provided in said lowermost zones as will be described later.

Furthermore, owing to the separation of the roof into two separated draining sections, the behavior of the roof is such that, if a weld fracture occurs in one of the said zones 6 or 7 and the consequent passage for the liquid contained in the tank above said floating roof, the following situations may ZHSCI l. Flooding of the zone 6 and partial immersion of the hollow annular member 3: the roof zone 7 which remains dry and the peripheral pontoon member 2 guarantee floating. The roof so constructed behaves as a roof having a central roof area of very small dimensions with a stress pattern which does not imperil the entire roof.

2. Flooding in zone 7, and in few compartments of the pontoon member 2 with partial immersion of the hollow annular member 4. In this case, the central zone 6 and all the remaining tight compartments of the pontoon member 2 collaborate in sustaining the roof. It is evident that the radial tractive forces which the membrane transmits to the peripheral pontoon member are very much reduced with respect to the simple pontoon roof. The reason for this is that the central membrane is sustained at many points by the nonflooded zones. What has been said above coupled with the smaller volume required for sustaining the roof (the external pontoon member must provide for sustaining a fraction of the roof surface) allow the adoption of a peripheral pontoon member with a reduced cross section.

The intermediate annular member 4 thus ensures a reduction of the free length of the membrane and provides a depressed zone on the roof, due to its weight, favoring a convergence of water towards the discharge holes 8 and 9 located in each of the zones into which the roof is subdivided near the annular member 4:. The mouthpieces of the drainpipes 10 for the rain water (FIG. 4) are fixed into these holes. These mouthpieces are covered by perforated screen plates 11 or the like, and the plurality of drainpipes are connected to a manifold 12 arranged beneath the floating roof. Said manifold 12 (one of these manifolds is provided for each plurality of drains 8 and 9 and their relative pipes 10) leads into collecting wells 13 (and E4) in which substantially syphon-type discharge members 15 are provided and are connected to flexible tubes 16 (and 17) of the ball-joint type for discharging the rain water to the outside of the liquid product storage tank 18. The two drains 8 and 9 are independent from one another.

Under certain conditions it may be advantageous to provide communication between the two draining zones and this is accomplished through conduit 49 provided with vaives 4b, arranged within the lower portion of the intermediate annular member 4, as visible in F166.

in the case of a failure of one of the two draining system valves 4b are opened thereby allowing the water or the liquid in general to be discharged nevertheless through the other draining system. The valve is actuated through particular apertures 4c normally closed by lids 4d, provided on the intermediate annular member 4 at regular intervals near the periphery thereof and above the respective valves.

Another purpose of the intermediate annular member is that of stiffening the roof against the stresses produced by the wind. This occurs because the roof plate is subdivided into two distinct surfaces by the intermediate annular member 4 and consequently the free lengths of the membrane are notably reduced.

According to a modified embodiment, there is provided a central floating caisson 5, having a circular periphery, with a diameter of far greater size than the transverse dimension of the members 2 and 4 and possibly subdivided into four caisson chambers (F l6. 1 and 7). This central caisson 5 is immersed in the underlying liquid in a manner similar to the peripheral pontoon member 2 and owing also to its size it is subjected to a buoyancy force which contributes therefore substantially to the overall floating of the roof. As a consequence to the buoyancy action acting on this central caisson, the central portion of the floating roof assumes a level higher than the level of the roof portion near the intermediate annular member 4, thereby improving the draining of flooding liquid towards the draining system.

The central caisson 5 has apertures 29 in the upper portion thereof, normally closed by suitable lids, the apertures being destined for inspection of the caisson chambers thereof.

When the central caisson 5 is uwd, the conditions described in point i of this description do not occur in practice, since it is improbable that all the caisson chambers of the central caisson 5 are damaged at the same time.

it will be understood that the intermediate annular member 4 acts also as a reinforcing structure for the roof.

The floating roof is further provided in its peripheral zone 7 with an annular element 20 of pressed sheet metal whose free ends are bent substantially in the formof a U-section iron. These bent ends, which are immersed in the liquid on which the roof floats, act as shock absorbers, while the annular element 20 constitutes a stiffener which stabilize the roof according to the invention and reduce its tendency to behave as a membrane.

A similar reinforcing element 19 visible in FIG. 2 is provided near the central zone in the embodiment without the central caisson 5.

The floating roof is further provided with a plurality of feet of adjustable height for resting the roof on the bottom ll8a of the tank 18. These feet are arranged in a regular annular concentric pattern and more precisely along a first annular line through the peripheral pontoon members 2 (feet 22), along two radially separated concentric circular lines in the external zone 7 close to the stiffening ring 20 (feet 23 and 23a), in the position of the intermediate floating element 4 (feet 24) and along an annular line in the said internal zone 6 close to the stiffening element 19 or caisson 5 (feet 25 All these feet consist of rodlilte elements 26 which slide inside tubelike elements 27 for their support and guiding and which are rigidly fixed by known means to the peripheral pontoon member 2, to the said external zone 7, to the intermediate annular member 4 and to the said internal zone 6.

The length of these feet is such to allow the zone near the intermediate annular member 4 to assume the lowermost level and allow the two-side draining even when the roof rests on the bottom of the tank through said feet. In fact, the feet 24 of the intermediate annular member are shorter than those of the peripheral pontoon member 2 and of the central caisson 5. Furthermore the feet 22 and 23a have almost equal length, which is of intermediate size with respect to those previously mentioned. These feets are staggered with respect to each other, so as to form sinoidal forms in the roof allowing a channeled draining of the flooding water or liquid towards the draining system even when the roof rests on the bottom of the tank through these feet, thereby preventing the formation of local flooding water pockets.

The invention as thus conceived may be subject to numerous variations and modifications within the scope of the appended claims.

lclaim:

1. A large diameter floating roof for liquid storage tanks, particularly for the storage of liquid petroleum products, inciuding a peripheral hollow annular pontoon member, a sheetlike member extending substantially over the entire roof area and fixed to said peripheral pontoon member, an annular load member arranged on said sheetlike member forming a depressed annular zone in an intermediate area concentric with the pontoon member and disposed radially inwardly thereof, said annular load member comprising a hollow annu lar member rigidly fixed on said sheetlike member adjacent said depressed annular zone and having continuous upstanding sidewalls to hydraulically separate said sheetlike member into two portions defining two distinct concentric draining sections, each of said draining sections having at least a draining hole in the range of said depressed annular zone, and further including a plurality of feet for supporting at least one of said portions of said sheetlike member defining said draining sections, said feet being arranged circumferentially spaced apart on two radially separated concentric circular lines and staggered with respect to each other, so as to confer a sinoidal fonn to said portion of said sheetlike element to allow draining of the roof even when the roof is supported on the bottom of said tank through said feet. 

1. A large diameter floating roof for liquid storage tanks, particularly for the storage of liquid petroleum products, including a peripheral hollow annular pontoon member, a sheetlike member extending substantially over the entire roof area and fixed to said peripheral pontoon member, an annular load member arranged on said sheetlike member forming a depressed annular zone in an intermediate area concentric with the pontoon member and disposed radially inwardly thereof, said annular load member comprising a hollow annular member rigidly fixed on said sheetlike member adjacent said depressed annular zone and having continuous upstanding sidewalls to hydraulically separate said sheetlike member into two portions defining two distinct concentric draining sections, each of said draining sections having at least a draining hole in the range of said depressed annular zone, and further including a plurality of feet for supporting at least one of said portions of said sheetlike member defining said draining sections, said feet being arranged circumferentially spaced apart on two radially separated concentric circular lines and staggered with respect to each other, so as to confer a sinoidal form to said portion of said sheetlike element to allow draining of the roof even when the roof is supported on the bottom of said tank through said feet. 